\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=1.0,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw (-3,0)node[left]{$A$} to [vR=$R_0$,*-*](0,2)node[above]{$B$}to [R=$R_x$,*-*](3,0)node[right]{$C$}--(-3,0);
  \draw (1,-1.5)to [battery2] (-1,-1.5);
  \draw (1,-1.5) to [cute open switch=$K$] (3,-1.5)--(3,0);
  \draw (-3,0)-- (-3,-1.5) to [pR=$R$,name=hb] (-1,-1.5);
  % \node at (hb.center)[below=2mm]{$R$};
  \draw(-.7,-1.5) --(-.7,-1.5|-hb.wiper)--(hb.wiper);
  \draw (0,2) to [rmeter, t=G] (0,.5);
  \draw[->] (0,.5)--(-.7,.5)--(-.7,0)node [below]{$D$};
  \node at (-1.85,0)[below]{$\ell_1$};
  \node at (1.15,0)[below]{$\ell_2$};
  % \node at (2,-2)[above]{$K$};
\end{circuitikz}
\end{document}